Molecular Self-Healing Processes in Polymers
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Molecular Self-Healing Processes in Polymers M. Chipara1, K. Wooley2 1 2
Indiana University Cyclotron Facility, Indiana University, Bloomington, Indiana Chemistry Department, Washington University in St. Louis, Saint Louis, Missouri
ABSTRACT A critical review of self-healing processes in polymers and composites based on polymeric matrices is presented. Two self-healing processes are analyzed, one “operating” within micron range and the other at molecular scale. Preliminary data on the molecular self-healing process in living polymers, as obtained by electron spin resonance are discussed. The possibility to develop a multi-scale self-healing material is suggested. INTRODUCTION The polymers and the composites based on polymeric matrices (CBPM) degrade slowly under the effect of various environmental factors (oxygen, UV, light, or temperature). This process, frequently identified as ageing, restricts the lifetime of polymers and composites materials. At sufficiently large stresses, local defects are generated; their accumulation triggers the formation of microcracks. Finally, the growth of microcracks results in a complete failure of the mechanical properties of polymers and CBPMs. Delamination processes reduces significantly the lifetime of complex composite structures. The adverse conditions of the space environment (such as radiation, extreme temperature, and micrometeorites) accelerate dramatically the degradation processes of polymers and CBPMs. The space exploration depends critically on the availability of lightweight materials. The reduced density of polymers justifies the efforts to design space materials based on polymers and polymeric matrices. Accordingly, it is mandatory to exploit the self-healing capabilities of polymers and CBPMs to reach the lifetime requirements for long term space missions. MICRON-SCALE SELF-HEALING PROCESSES IN POLYMERS AND CBPMS The “classical” composite with self-healing capabilities (CSHC) consists of a polymer, which contains a catalyst and micron-sized bubbles filled with monomer. This CSHC is based on the dispersion of about 2.5% Ru based catalyst (Grubbs) and of micron sized spheres filled with dicyclopentadiene , DCP, [1-3] (see Fig. 1) within polymeric matrices. The microencapsulation of DCP in poly-urea formaldehyde was performed in an oil-in water emulsion. The diameter of microcapsules is ranging from 10 to 10,000 µm [1-3]. To trigger the self-healing features, the stresses have to tear the microcapsule, and to release its content within the polymer. The ring opening metathesis polymerization of DCP catalyzed by the Ru derivative will result in the formation of a cross linked polymer that will fill microcracks and will delay the polymer failure. The requirements for efficient self-healing capabilities are: 1. The microcapsules have to survive to polymer processing. 2. The local stress required to break the microcapsule should be as low as possible. 3. The catalysts dispersed within the polymer should not initiate polymer degradation. 4. The dispersion of fille
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